Wednesday, March 2, 2016

Eyes Under Puget Sound: Critter of the Month - Slender Sea Pen

By: Dany Burgess & Angela Eagleston, Environmental Assessment Program

Where there’s a quill, there’s a way

This month’s critter looks a lot like an old fashioned-feather quill pen, and is fittingly named Stylatula elongata, the sea pen.

This critter of the month, Stylatula elongata, is often found in groups.
Photo courtesy of Chris Grossman, www.diver.net

Sea pens are a type of octocoral, or soft coral, which are related to jellyfish and anemones. They may not get as much love as their glamorous cousins, the reef-building hard corals, but sea pens are fascinating creatures that can live to be 100 years old, glow in the dark and live right here in the soft sediments of Puget Sound!

Many heads are better than one

A sea pen is not one single animal, but a colony of many tiny animals called polyps. Each colony contains several types of polyps which contribute to its survival in different ways. The feeding polyps, called autozooids, wave their tentacles in the water column to catch drifting plankton. Siphonozooids, a different kind of polyp, are responsible for distributing food throughout the colony using a complicated water circulation system.

Glow with the flow

Although sea pens may appear to be inanimate plant-like organisms yielding to the will of the water currents, they actually have a surprising number of defensive tricks up their sleeves. If disturbed, the polyps can take in water, inflating the colony so it can drift to a new location. They can also quickly expel water, deflating the colony and allowing it to retreat into the mud.

S. elongata autozooid polyps with tentacles extended.

Another way sea pens respond to stimulation is by glowing in the dark, through a process called bioluminescence. Bioluminescence occurs when a chemical reaction takes place within the sea pen and energy is released in the form of light. These showy displays resemble the lights on a movie marquee as the flashes circulate up and down the rod-like stem of the sea pen, and are used to startle potential predators.

LEFT: Part of S. elongata with the supporting skeletal rod exposed.
RIGHT: Close-up of polyp clusters with needle-like spokes called sclerites.

In rod we trust

Like other sea pens, S. elongata is supported by a hard white skeletal rod made of calcium that looks like a lollipop stick. At the base of the rod is a muscular, bulbous holdfast called the basal peduncle that anchors it in the soft sediment.

Arranged in spirals around the rod are clusters of polyps which create the sea pens’ feather-like appearance. S. elongata can be distinguished from other species of sea pens by the exposed fans of needle-like spikes called sclerites which protect the soft, fleshy polyps from its two main predators, nudibranchs (sea slugs) and sea stars.

A muscular, bulbous holdfast called the
basal peduncle anchors S. elongata in
the soft sediment in which it lives.

Sea pen on the decline?

Despite having so many strategies for survival, sea pens are on the decline in some parts of Puget Sound. Scientists aren’t sure why this is happening, but their long lifespan (100+ years) may be considerably shortened by disease, pollution, disturbance and other unknown factors.

In each issue we will highlight one of the Sound’s many fascinating invertebrates. We’ll share details on identification, habitat, life history, and the role this critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.

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